Many factors have been documented that can contribute to mortalities observed in finfish aquaculture including disease and harmful algal blooms of cyanobacteria, in addition to more common issues with oxygen stress or nitrogen toxicosis. The source, occurrence, and epidemiology of many freshwater, estuarine, and marine toxins produced by algae are well known. For instance, divisions of photosynthetic plankton are known to produce toxins that include but are not limited to Bacillariophyceae, Pyrrophyta, Prymnesiophyta, Raphidophyta, as well as certain members of the cyanoprokaryota. Impacts from these toxins are dependant on the affected organism, as well as route, concentration, and duration of exposure.
While cyanoprokaryotic algae, diatoms, prymnesiophytes, dinoflagellates, euglenoids, and rhaphidophytes are long known to produce algal toxins, the identification of a toxic euglenoid is unexpected given that this species of Euglena that was identified by Ehrenberg in the 1830s has presented no conclusive evidence of toxin production. An exception would be a tilapia-kill event detailed in Xavier M B, et al., 1991. Algological Studies, 62:133-142, wherein tilapia exposed to a Euglena sanguinea bloom in aquaria had euglenoid cells associated with gills, resulting in distressed breathing as manifested by surface porpoising and minor tilapia fish mortality.
Euglena form protective cyst when subjected to hostile environments as a survival mechanism. This formation contributes to the difficulty in recognizing toxins produced by euglena as these cells encyst when water is turbulent. Other environmental factors contribute to difficult toxin identification. One scenario is that a surface scum of the euglenoid forms in calm weather during mid-morning to afternoon, resulting in high concentrations of toxin in several centimeters thickness of water. Wind events would result in dissipation of the scum through encystment, leaving a surface microlayer containing dissolved toxins. Aquacultured fish are then fed floating feeds, resulting in concentrated exposure. These events lead to an increased difficulty in identifying a euglenophycin as the source of a toxin.
While cyanoprokaryotic algae, diatoms, prymnesiophytes, dinoflagellates, euglenoids, and rhaphidophytes are long known to produce algal toxins, euglenoid algae that produce toxins were isolated from aquaculture ponds, with toxin confirmation based on positive fish bioassays following exposure to the isolated clonal algal cultures. It remains an open question as to the isolation of toxin from euglenoid algae blooms at freshwater facilities.
Furthermore, while taxonomists have recognized the presence of euglenoid algae in both freshwater and marine systems, the lack of unique pigment biomarkers have prevented routine monitoring using remote sensing methodologies or HPLC pigment biomarker identification would lead to under-estimation of importance of the division. Additionally, since the existence of a euglenoid toxin was only recently reported many previous fishkills caused by unidentified biological agents could be attributable to euglenoids. The apparent potency of this compound strongly suggests further assessment of occurrence in potable waters.